Microbial Pollution Characterization at a TMDL Site in Michigan: Effect of Hydrological Conditions on Pollution Loading

Communities throughout the United States are developing and implementing watershed management plans to address nonpoint sources of pollution and meet Total Maximum Daily Load (TMDL) requirements. Once a TMDL is established, a watershed management plan is developed and implemented to reduce contaminant sources and attain TMDL goals. Developing an effective TMDL and remediation plan should take into account fluctuation of pollution loadings and the timing of first-flush events. The objective of this study is to investigate the effect of hydrological conditions on microbial pollutant levels at a TMDL site during spring and summer storm events. A total of 64 water samples were collected from Sloan Creek in mid-Michigan in the spring/summer of 2015. All samples were analyzed to quantify concentrations of E. coli, bovine-associated Bacteroides (BoBac) gene markers, and human-associated Bacteroides (HuBac) gene markers. Discharge was the driving force of microbial contaminant loading in the studied water body. E. coli concentrations had significant strong correlation with precipitation and discharge, and BoBac concentrations were positively related to discharge. E. coli, BoBac and HuBac patterns suggested first-flush phenomena occurred during summer storms. E. coli permit exceedance rates increased from 31% before first-flush, to 100% during and after first-flush in the summer. The resulting information may help develop a plan for restoring impaired waters and establish the maximum amount of pollutants that the body of water can receive during different hydrological conditions.